In the same way, a well conducted poll of 1,000 people can, most of the time, give us an idea of what the country as a whole is thinking.

However, there are several problems that pollsters need to overcome to have a chance of accurately reflecting the whole electorate.

Margin of error

The first caveat is that no poll can be 100% correct 100% of the time. Polling companies generally claim that 95% of the time, a poll of 1,000 people will be accurate within a margin of error of +/-3%.

This means that a figure in the poll could be up to three percentage points higher or lower than that shown.

So if the Tories are on 32% and Labour is on 38%, there is a chance they could both be on 35%.

It is, however, more likely that the figures will be 1% out rather than 3%.

The question

How the question is framed can have an effect on the results. Companies have to decide whether to simply ask people who they would vote for or to remind them of the choices (as they would have on a ballot paper).

Research indicates that the Lib Dems have a higher rating in polls where respondents are given a list of parties to choose from.

Weighting

Another issue is how to ensure the sample is representative of the general population. To achieve this, polling companies "weight" their data to match the demographic profile of the UK.

At its most basic level, this means that if a poll of 1,000 people is made up of 550 men and 450 women, it is unrepresentative because it does not reflect the profile of the UK population (51% female).

So the answers of female respondents will be given slightly more weight (in this case they will each count as 1.133 people) to give them a representative impact on the final findings.

Conversely, the men will be weighted to each count as 0.891 people.

The same procedure is routinely carried out for age group, social class and region.

Past vote

For voting intention polls, further adjusting is required. Some pollsters weight by past vote - they ask respondents who they voted for last time and weight the sample so that it more closely matches the political make-up of the general population.

One problem with doing this is that a certain number of people will incorrectly recall who they voted for last time - and a few will even lie about it.

So the polling companies we have included so far that weight by past vote - ICM and Populus - use a variety of further methods to improve the accuracy of their weighting.

These include using information from a range of previous voting intention surveys.

Likelihood to vote

Most companies then weight or filter by likelihood to vote so that the answers of people who are most likely to vote are given the most prominence in the results.

This does have the effect of reducing the number of people on whose answers the final voting intention figures are based - which in turn raises the effective margin of error.

'Shy' Voters

Finally, several pollsters reallocate a percentage of "don't knows" to the party they voted for last time.

This is to get around the problem that emerged following the 1979 election with the phenomenon of "Shy Tories" or the "spiral of silence" - people who do not like to admit they support a certain party but who vote for them nonetheless.

Whilst initially this particularly applied to Conservative voters, some have suggested that it may apply to some "bashful" Labour voters this time.

Timing of publication

The date for each poll in the poll tracker is the date on which it was published. Where two polls appear on the same day, the 'default' poll shown is the one with the most recent fieldwork.

ComRes

Question: If there was a general election tomorrow, would you vote Conservative, Labour, Liberal Democrat or for some other party.

The names of the three major parties are rotated in the question at random.

If respondents decline to name a party in response to the voting intention question, they are asked how they would probably vote if it were a legal requirement to do so and allocated this party.

All who continue to remain undecided or refuse to say for whom they would vote are then allocated a party, according to the party with which they most closely identify. In both cases, the data is weighted by reported likelihood of voting.

Interview method: Telephone

Sample size: Approximately 1,000 adults age 18+

Sample method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is demographically weighted to reflect the profile of all adults in the UK aged 18+, including those in households that do not own telephones.

ComRes weights the data by likelihood to vote. This is done by asking respondents how likely they are to vote (on a scale of 1-10 - with 10 being absolutely certain to vote).

The answers of respondents who answer four or below are excluded. The rest are weighted proportionately so that someone who answers 10 gets the most weight and someone who answers five gets the least.

Respondents who have answered "don't know" to the voting intention question or refuse are asked a further "squeeze" question: if it were a legal requirement for you to vote, which party do you think you probably would vote for?

This extra data is then weighted and added to the original voting intention results.

To ensure the sample is politically representative, ComRes weight by how respondents recall having voted at the last general election. This weighting is based in part on comparing the distribution of this recall vote (for those who declare for whom they voted) with the actual result of the 2005 General Election and in part with the average past vote obtained in their previous twelve polls. Three-quarters of the weight is based on the former comparison and one-quarter on the latter.

Question: "If the general election were held tomorrow for which party do you intend to vote?", all major parties listed. If respondent undecided then "Well if you had to say, which party are you most inclined to support?" and if party selected then 'squeezed' into voting intention scores.

Interview method: Online via Harris Interactive's online panel

Sample size: Approximately 1,000 likely voters

Sample method: Outgoing sample stratified by age, gender and region

Weighting: Polling data is weighted by matching the distribution of several demographics with the known distributions in the population.

The demographic variables used are age, gender, education, region of the UK, and internet usage.

All of the target information is taken from official UK statistic, except for internet usage which is compiled from Harris Interactive's own surveys.

The reason for including Internet usage - even though the survey is intended to be representative of the general population - is to correct for the fact that those who are online more often are more likely to respond to an email invitation.

Another method - propensity scoring - is used to counterbalance the selection bias caused by using an online survey, where respondents have chosen to be online, chosen to join a panel, and chosen to respond to that particular survey.

Propensity scoring is essentially a measure of how likely an individual is to be in a particular sample based on their attitudinal, behavioural and demographic characteristics.

Question: If there were a general election tomorrow which party do you think you would vote for? (Conservative, Labour, Liberal Democrat, Plaid Cymru/SNP, other party)

Interview method: Telephone

Sample size: Approximately 1,000 adults age 18+

Sample method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is demographically weighted to reflect the profile of all adults in the UK aged 18+, including those in households that do not own telephones.

ICM then weights the data by past vote.

The answers are also weighted by likelihood to vote. This is done using a combination of how likely respondents say they would be to vote in a new election (on a scale of 1-10) and how consistent they have been at voting in the past.

In a further step, ICM takes 50% of those who intend to vote and have a good voting record but are undecided who they will vote for this time and allocates them to the party they say they voted for in 2001.

Question: "How would you vote if there were a general election tomorrow?" All 'undecided' and 'refused' responses are then asked a 'squeeze' question: "Which party are you most inclined to support?" and responses from the two are combined. (Conservative, Labour, Liberal Democrat, [SNP/Plaid Cymru in Scotland/Wales respectively], Other)

Interview method: Telephone

Sample size: Approximately 1,000 British adults aged 18+ by telephone

Sample method: Random digit dialling (so as to include households with ex-directory numbers), with sample stratified by Government Office Region (GOR). Respondents are selected on a quota basis to reflect the demographic make-up of the population.

Weighting: Data is weighted to reflect the profile of the population, using the latest census data and recent updates from the Office for National Statistics and other surveys. Data is weighted by age, gender, social grade, work status, public vs. private sector worker, region and car ownership. Ipsos MORI does not weight by past vote.

Certainty of voting filter: Ipsos MORI asks respondents to rate their likelihood to vote on a scale of 1-10 where 10 means absolutely certain to vote and 1 means absolutely certain not to vote. The data is then filtered so that only the responses of those who answer 10 out of 10 (absolutely certain to vote) make up the final figures.

Question: "If there was a general election tomorrow, which party would you vote for? Would it be [rotate order] Conservative, Labour, Liberal Democrat, SNP [Scotland only], Plaid Cymru [Wales only] or another party?"

If 'another party':

"Would that be [rotate order] the UK Independence Party (UKIP), the Green Party, the British National Party (BNP), or some other party - or do you not know how you would vote?"

Sample: Approximately 1,500 adults 18+

A more detailed explanation of the methodology involved can be found on the
Populus website.

Sampling method: Within each government office region a random sample of telephone numbers was drawn from the entire BT database of domestic telephone numbers. Each number so selected had its last digit randomised so as to provide a sample including both listed and unlisted numbers.

Figures are adjusted for turnout on the basis of respondents' declared likelihood of voting.

In a further step Populus weights the whole sample on the basis of its 'past vote' - adding the most recent poll data to its previous 20 most recent voting intention polls (so as to avoid the random volatility that can appear in comparing any two individual samples) and calculates the past vote weighting from the average recalled past vote in this data, giving a weight of 50% to the actual result of the last election and 30% to the average recalled past vote from the most recent polls.

An additional final step is then taken to address the tendency for 'spirals of silence' among supporters of unpopular parties causing an inadvertent bias in voting polls.

This is done by taking those respondents who will disclose which party they voted for at the last election, but refuse to answer the question of how they would vote in an election now, or say they don't know. They are then reallocated to the party they voted for at the last election on the basis of values derived from a 5,000 sample callback poll immediately after the last election, when Populus re-interviewed people polled in the run up to the election.

Their responses are compared in terms of what they actually did with what they said they would do when first interviewed.

YouGov

Question: If there were a general election held tomorrow, which party would you vote for?

Sample method: YouGov has recruited an online panel of nearly 300,000 subscribers. From this panel YouGov selects a sub-sample to match the Great Britain electorate by age, gender, social class, newspaper readership and party ID.

Only this sub-sample has access to the questionnaire. Respondents are paid for taking part.

Weighting: Data is weighted to the profile of all GB adults aged 18+ including people without internet access.

As well as weighting by gender, age, social class and region, YouGov weights by readership of individual newspapers and party ID.

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